The invention relates to a hybrid gas generator, in particular for filling a gas bag, for example, an air bag for an automotive vehicle.
Hybrid gas generators are known in various embodiments. A hybrid gas generator is characterized by a gas generating substance, mostly in the form of a pyrotechnic propellant charge provided in a combustion chamber, and by a gas vessel which contains a suitable stored gas such as argon with a small portion of helium. The solid or liquid gas generating material may be activated via an ignition unit, which is mostly configured to be electrically controllable, with gas being generated in the combustion chamber which contains the gas generating material. In known hybrid gas generators, the combustion chamber, which is usually sealed off from the gas vessel at the beginning, is configured such that the gas generated in the combustion chamber actuates a unit which opens the combustion chamber and the gas vessel in order to allow the gas generated in the combustion chamber and the stored gas contained in the gas vessel to escape. Preferably, the two types of gas are mixed with each other on this occasion.
A hybrid gas generator of this kind is known from the document DE 196 02 009 A1, for example. This gas generator comprises a combustion chamber with an outlet opening in which a projectile is arranged. As soon as a predetermined pressure is exceeded in the combustion chamber, the projectile held in the outlet opening by positive locking or via an adhesive bond is ejected. The combustion chamber is coaxially arranged inside a gas vessel for the stored gas, and the gas vessel comprises a bursting membrane in the direction of ejection of the projectile, which can be destroyed by the projectile. In this way, a mixture of the gas generated inside the combustion chamber and of the stored gas contained in the gas vessel escapes from the outlet opening of the gas vessel.
What is disadvantageous in this embodiment of a hybrid gas generator is the fact that the response time passing between the ignition of the gas generating material in the combustion chamber and the escape of gas from the gas vessel is not defined exactly, because the response time particularly depends on the threshold pressure at which the projectile is ejected from the outlet opening of the combustion chamber. Usually, holding the projectile by positive locking or glueing it to the outlet opening does not make it possible to keep a defined threshold pressure, at which the projectile is ejected, with the required exacting tolerances.
Furthermore, the coaxial arrangement of the combustion chamber inside the gas vessel is disadvantageous whenever a design of the gas generator is required which is slim, but extended in the axial direction.
From the document EP 0 776 800 A2, a hybrid gas generator is known in which the gas vessel with the stored gas is arranged adjacent the combustion chamber in the axial direction. Here, too, the outlet opening of the combustion chamber is closed by a projectile, which is acted upon by the pressure of the gas generated inside the combustion chamber. The gas vessel also comprises a bursting element in the direction of flight of the projectile, which is destroyed when it is hit by the projectile and unblocks the outlet opening of the gas vessel.
With this embodiment of a hybrid gas generator, it is to be assumed that the axial extent of the gas vessel may only be relatively small, as otherwise no sufficient hitting accuracy of the projectile on the bursting element can be guaranteed.
In this reference, the use of an additional metal foil for sealing the outlet opening of the combustion chamber is mentioned, with this foil being provided on the back side of the projectile and connected with the corresponding wall of the combustion chamber. Apart from this sealing function, no other indications regarding the function of this foil are to be found in this reference.
An object of the invention is to provide a hybrid gas generator, in particular for filling a gas bag, which guarantees a high degree of functional safety and a very fast and defined response time.
The invention is based on the idea that a membrane for closing the outlet opening of the combustion chamber can be dimensioned in such a way that a destruction of the membrane as a result of the pressure of the gas in the combustion chamber acting thereon is guaranteed within very exacting tolerances around a predetermined threshold pressure. As a result, a projectile provided in a guide channel is not acted upon by pressure until the threshold pressure inside the combustion chamber has been exceeded. For this reason, there is no risk that the projectile is moved out of its starting position as a result of the pressure of the gas generated in the combustion chamber below the threshold pressure or that the forces fixing the projectile in the guide channel are influenced. Thus, the influence of all ignition or inflaming processes on the projectile is eliminated. Accordingly, the response time passing until the projectile is ejected from the guide channel, depending on the increase in pressure inside the combustion chamber, can be kept in very narrow bounds. As the barrel length (i.e. the effective length of the guide channel guiding the projectile until it is ejected, which acts as a barrel for the projectile) determines the final speed of the projectile together with the pressure acting thereon in the guide channel, the response time passing until the outlet membrane of the gas vessel is destroyed can be exactly met. The accuracy of the trajectory of the projectile can be further increased if the guide channel is provided with grooves similar to a gun barrel.
In another embodiment of the invention, no projectile is separately provided in or at the outlet opening of the combustion chamber or in the guide channel. In this case, a membrane is used which is configured such that the part of the membrane closing the cross-section of the guide channel is broken out of the membrane substantially in one piece if a predetermined threshold pressure of the gas generated in the combustion chamber is exceeded; the part that has been broken out acts as a projectile. The part broken out may be thickened compared to the residual portion of the membrane. In any case, it must be ensured that the part broken out has such a mass that the kinetic energy of this projectile is sufficient to destroy the outlet membrane of the gas vessel when the projectile hits it.
This embodiment has the advantage of a very easy manufacture of the hybrid gas generator.
The membrane may be provided at the end of the guide channel on the side of the combustion chamber or inside the guide channel; the portion of the guide channel between the position of the membrane and the end of the guide channel serves as a barrel for guiding the part of the membrane that forms the projectile. Hereby, the final speed of the projectile may be kept in very narrow bounds.
The membrane may be acted upon by shearing edges on the front side opposite to the combustion chamber. Hereby, breaking out of the membrane part within the cross-section of the guide channel in one piece is facilitated. This, too, contributes to the breaking out of the membrane part forming the projectile within exacting tolerances around the desired threshold pressure.
The membrane may also be provided on the front side of the guide channel opposite the combustion chamber.
In this case, the barrel length is zero. For this reason, this embodiment is the obvious choice particularly for short distances between the membrane from which the projectile is broken out and the outlet membrane to be destroyed, because in this case, as a result of the barrel length of zero, the hitting accuracy and the time of flight of the projectile until it hits the outlet membrane is reduced as compared to the embodiments mentioned before. The problem of the hitting accuracy can be solved by providing guiding surfaces which guide the projectile towards the outlet membrane to be destroyed, if this should be necessary.
According to an embodiment, the membrane from which the projectile is to be broken out may be provided with a corresponding predetermined breaking point which has a closed, ring-shaped configuration. This breaking point may be a closed, ring-shaped groove or notch, for example. In the embodiment in which the membrane is arranged on the front side of the guide channel opposite the combustion chamber, the membrane may be connected with the corresponding front side of the guide channel by means of a ring-shaped weld. In this case, the edge portion of the weld on the inside constitutes the predetermined breaking point. Here, it is possible to break out a portion forming the projectile from the membrane which is larger than the cross-section of the guide channel. According to the invention, the cross-section of the guide channel is preferably set to a value which ensures that the maximum working pressure generated in the gas vessel after the gas generating material has been activated is below a specific value, preferably lower than 300 to 400 bar.
According to another embodiment of the invention, the outlet membrane may be configured and designed such that it is destroyed as soon as a critical pressure of the mixture of stored gas and gas generated is exceeded in the gas vessel even without being destroyed by the projectile. This guarantees the operational reliability of the gas generator, as the gas generator is triggered even if the outlet membrane of the gas vessel is not destroyed by the projectile because of malfunction.
The wall thickness of the gas vessel may be designed for the maximum working pressure (in case of destruction of the outlet membrane by the projectile) or the critical pressure at which the outlet membrane is destroyed, also considering a safety reserve. This leads to a decrease in weight and manufacturing costs of the gas generator without impairing the operational reliability or safety thereof.
In the preferred embodiment, the gas vessel may be tube-shaped. The guide channel may be provided in a disk-like end piece that is connected with the corresponding end of a tube or is integrally formed therewith.
The combustion chamber, too, may be tube-shaped; preferably, a tube section forming the housing of the combustion chamber is connected with the end piece of the gas vessel or is integrally formed therewith. Hereby, a very slim design is achieved with little constructional efforts.
According to a further embodiment of the invention, the outlet opening of the combustion chamber may be shielded from the solid or liquid particles produced when gas is generated by means of a shielding element. The shielding element is arranged in front of the outlet opening of the combustion chamber at a predetermined distance thereto.
The shielding element avoids that the solid or liquid particles are ejected directly without reducing their speed. The particles can then only emerge from the outlet opening of the combustion chamber if they are entrained in the stream of gas which is forcibly deviated.
In an embodiment of the invention, the shielding element may be configured as a sieve or filter. Hereby, it is at least avoided that solid or liquid particles exceeding a specific size will emerge.
Preferably, the shielding element comprises a rebounding or impact portion which covers the outlet opening of the combustion chamber in such a way that a direct, straight entrance of solid or liquid particles into the outlet opening of the combustion chamber without reducing the speed thereof or their direct striking of the membrane is avoided.
In a further embodiment of the invention, the portion surrounding the outlet opening of the combustion chamber and/or the rebounding portion of the shielding element, which are subjected to being hit by solid or liquid particles, may consist of a material in which some of the particles penetrate and remain trapped and from which some of the particles rebound, potentially after having burst once they hit the aforesaid portions. By this, it is achieved that large particles are either crushed and obtain a size that does not pose any problems or remain trapped.